Polymorph of 2,9-dichloroquinacridone and its electronic properties

The title compound (2,9-DClQA) is a hydrogen-bonded, bluish-red pigment. However, red and black single crystals have been isolated from the vapor phase in the higher- and lower-temperature regions around 380 and 150 degrees C, respectively. Because of this, correlation has been studied in the present investigation between the crystal structure and the shade in the solid state. The red phase is found to correspond to the commercial product as characterized by two-dimensional NH...O hydrogen bonds between the NH group of one molecule and the O atom of the neighboring one. On the other hand, the molecule of the black phase is heavily deformed and no intermolecular hydrogen bonds are recognized. The molecular distortion induced by crystallization in the black phase is found to displace the absorption band toward longer wavelengths to bring about the black color. In addition, the black phase is observed to be transformed into the red one around 200 degrees C.